Multi-scale drivers shape bird taxonomic and functional diversity in Mediterranean urban forests

Abstract Urban green spaces provide critical habitat and support biodiversity within cities, and birds serve as sensitive bioindicators of environmental quality. Identifying which urban drivers most strongly affect biodiversity is essential for planning resilient and eco-friendly cities. This study aimed to identify the relative contribution of local forest and landscape-scale factors in shaping taxonomic and functional dimensions of urban bird diversity, and to determine whether different biodiversity components respond to environmental drivers operating at different spatial scales. The taxonomic and functional diversity of urban forest bird assemblages was investigated across 12 urban forests of Florence, Rome, and Campobasso (Italy), examining species richness, abundance, and functional dispersion in relation to local- and landscape-scale environmental predictors. Random Forest models were applied to evaluate the relative importance of both individual predictors and aggregated effects of local and landscape variables, with SHAP analysis used to interpret the direction and strength of associations between key predictors and model predictions. Results revealed scale-dependent patterns, with species richness and functional diversity primarily influenced by local forest attributes, including tree density, crown ratio, and vertical structural heterogeneity, suggesting that stand-level habitat complexity enhances niche availability. In contrast, bird abundance was more strongly linked to landscape-scale variables, such as forest aggregation and land-cover diversity, indicating that broader spatial context facilitates movement, dispersal, and resource access. Non-linear relationships indicated that bird diversity was highest under intermediate levels of landscape heterogeneity and forest aggregation within the range observed across study sites, suggesting that balanced landscape configurations may provide complementary resources and habitat opportunities. These findings underscore the importance of multi-scale approaches to urban biodiversity conservation, demonstrating that both fine-scale habitat structure and landscape connectivity are associated with variation in urban bird communities and provide guidance for enhancing management practices in urban forests.